Base poll adjustment on number of sourcestats samples

Instead of following skew changes, adjust polling interval so that the
number of measurements used in the regression algorithm remains close to
a target value. It can be configured with a new polltarget option
(6 by default).

sourcestats.c

@@ -789,6 +789,14 @@ SST_Skew_Direction SST_LastSkewChange(SST_Stats inst)
 
 /* ================================================== */
 
+int
+SST_Samples(SST_Stats inst)
+{
+  return inst->n_samples;
+}
+
+/* ================================================== */
+
 void
 SST_DoSourcestatsReport(SST_Stats inst, RPT_SourcestatsReport *report, struct timeval *now)
 {